1) Field of the Invention
The present invention relates to an optical transmission apparatus, a continuity testing method therein, and an optical transmission system suitable for use in an optical communication system having the wavelength division multiplexing (WDM) function.
2) Description of the Related Art
In the field of the optical communication networks having the WDM function, the DWDM (Dense WDM) technique is introduced into not only known long-distance (long-haul) networks but also local (metro) networks because of a recent rapid increase in communication traffic. In the WDM network, an optical add drop multiplexer (OADM) for adding a channel to be wavelength-multiplexed or dropping a channel is appropriately arranged as a node.
Particularly, many WDM networks in the metro networks adapt a network configuration in which a number of OADM nodes are arranged in a ring. With expansion of the network scale in the future, it is estimated that the number of the OADM nodes to be arranged in a ring is increased.
FIG. 11 is a diagram showing a structure of a general OADM node used in the above-described WDM network. The OADM node 100 shown in FIG. 11 comprises a pre-amplifier 101, an optical demultiplexer 102, an optical crossconnect unit 103, a variable optical attenuator 104, an optical multiplexer 105 and a post-amplifier 106, as optical components.
A wavelength-multiplexed signal inputted to the OADM node 100 through a transmission line 107 is amplified by the pre-amplifier 101, then demultiplexed into wavelengths by the optical demultiplexer 102. The optical crossconnect unit 103 selectively outputs each of wavelength components demultiplexed by the optical demultiplexer 102 toward either the VOA 104 in the following stage or a drop port DP. When the wavelength component demultiplexed by the optical demultiplexer 102 is outputted to the drop port, a light beam of a corresponding wavelength component can be inputted to the optical crossconnect unit 103 through an add port AP, and outputted to the VOA 104.
The VOA 104 variably attenuates each of the wavelength components outputted from the optical crossconnect unit 103 to adjust the level thereof. The optical multiplexer 105 wavelength-multiplexes light beams of the wavelength components from the VOA 104. The post-amplifier 106 amplifies the wavelength-multiplexed beam from the optical multiplexer 105 as needed, and outputs the wavelength-multiplexed signal beam undergone the add/drop process through a transmission line 108.
In Patent Document 1 below, there is described a technique in which a function corresponding to the above optical crossconnect unit 103 is configured with 2×2 optical switches in number corresponding to the number of wavelengths demultiplexed by the optical demultiplexer 102, and the VOA 104 has an output constant controlling function of automatically keeping the power of each wavelength component at a predetermined constant value.
As shown in FIG. 11 described above, the OADM node 100 is configured with various optical components (refer to reference characters 101 to 106). It is general to adopt optical connectors to optically connect these optical components. Adoption of optical connectors for the connections may increase the splice loss or the quantity of reflectance because of contaminants adhering on the spliced surface of the connector. When the splice loss or the quantity of reflectance is considerable, it may cause disconnection due to degradation of the transmission quantity such as the bit error rate.
On the other hand, with an increase in the number of wavelengths and nodes arranged in a ring, the number of connection points by the use of optical connectors as being the OADM nodes 100 is rapidly increased. When the number of wavelengths is 40 and the number of OADM nodes arranged in a ring network is 20, for example, the number of connection points inside the node by the use of optical connectors or the like in the ring network is considered to be more than 3000 only in the OADM nodes.
Accordingly, the continuity test on a point of the optical connector which may cause an error is important to confirm the performance of the apparatus. However, if the continuity test is performed on each of all the points at which the connectors are mounted, the number of times of the operation will be considerable because the number of the points is considerable. With respect to an optical apparatus other than the OADM node, the load of the operation may be enormous at the time of the continuity test on the apparatus that has a function of demultiplexing wavelengths of a wavelength-multiplexed optical signal.
With respect to the above problem, the number of wavelengths at the time of a start of the operation is, practically, about ten at most when WDM is introduced into a real line. Accordingly, only in the continuity test on optical propagation paths through optical components disposed for communications over channels to be used at the time of the initial start, the continuity of signals is confirmed (continuity confirmation) by using a transceiver such as a transponder, which is introduced and connected along with the WDM system.
In concrete, in order to output a light beam having a wavelength of each channel to an optical propagation path to confirm the continuity, an optical output element such as a wavelength tunable laser or the like is separately disposed, and the receiving state of the light beam propagated through the optical propagation path is confirmed by means of a transponder or the like while the output light beam wavelength of the wavelength tunable laser is appropriately controlled, whereby the continuity of the optical propagation path of each wavelength channel is confirmed.
In the method using a transponder to confirm the continuity, it is unlikely that a transponder for a channel not used at the time of the initial operation is disposed. For this, it is not general to confirm the continuity of an optical propagation path through optical components disposed for communication over a channel not used at the time of the start of the operation.
In this case, when an operational channel is increased because of an increase in traffic, the continuity test using a transponder newly disposed is performed after the operation of the apparatus is started. When the confirmation operation reveals abnormality in the continuity of the optical propagation path through optical components disposed for communication over an individual optical channel, it becomes necessary to do a continuity restoration work such as cleaning of an optical connector connecting optical components for a relevant channel.
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 2000-4213
However, in a system using an optical amplifier for collectively amplifying wavelength-multiplexed signals, the maintenance work such as cleaning or the like of a connector spliced surface requires all the channels to be once stopped, in many cases. Namely, it is necessary to stop the communications over the other channels in which no error occurs each time an operational channel is increased in order to do the necessary continuity restoration work for the increased channel. It is desirable to avoid such stopping of the communication channels in operation each time a channel is increased, as much as possible to keep the stability of the communication.
In order to decrease the possibility that such cleaning work on a connector becomes necessary after the operation is started, it is desirable to collectively test the continuity of optical propagation paths of all channels that can be accommodated when the system is introduced. When the apparatus is operated as a node, it is desirable to detect abnormality in each optical component that can be hindrance of the continuity of the light beam on the optical propagation path, as well as the continuity confirmation of the optical connectors, as a matter of course.
There have been problems that it is necessary to separately prepare an element such as a wavelength tunable laser that can output a light beam having each channel in order to test the continuity, that the cost increases when the continuity testing function is given to the node apparatus, and that a longer time is required until the commercial operation is started because the load on the operator who controls the wavelength tunable laser increases.